1. Field of the Invention
Embodiments of the subject matter disclosed herein generally relate to methods and devices performing a model based anti-surge dead time compensation in systems including a compressor.
2. Description of Related Art
Since oil and natural gas remain a source of energy that cannot be replaced at a significant enough proportion in the world economy, the interest in developing new production fields has continued to increase. Compressors are frequently used in pipelines for transporting the natural gas from a production site to consumers, in oil refineries, refrigeration systems, gas turbines, etc. In a compressor, a pressure of a fluid flow is increased by adding kinetic-energy/velocity to the fluid flow, for example, through rotation of a rotor or an impeller inside the compressor.
A compressor's operation may be affected by the occurrence of a surge phenomenon. The surge phenomenon occurs when the compressor cannot add enough energy to overcome the system resistance, which results in a rapid decrease of the flow and the discharge pressure. The surge occurrence may be accompanied by high vibrations, temperature increases and rapid changes in an axial thrust, which may damage the compressor. Repeated and long lasting surges may result in catastrophic failures. Most systems including compressors are designed to detect a surge trend and to operate to reverse the surge trend. For example, in order to reverse a surge trend, the fluid flow through the compressor may be adjusted by modifying an amount of fluid recycled through the compressor.
FIG. 1 is a schematic diagram of a conventional system 1 including a compressor 10. The system 1 includes an anti-surge loop 20 through which a part of the fluid output at an outlet 22 of the compressor 10 may be recycled to an inlet 24 of the compressor 10. The amount of fluid recycled via the anti-surge loop 20 depends on an actuator position of an anti-surge valve 30 located along the anti-surge loop 20. An anti-surge controller 40 controls the anti-surge valve 30, thereby determining amount of fluid recycled. The flow through the compressor 10 is modified by modifying the amount of fluid recycled. The compressor 10 receives fluid from an expander 42. Fluid line sensors and fluid handling components are usually present along an anti-surge loop, but FIG. 1 represents a minimal set of elements relevant to the current discussion.
A time delay occurs between when the anti-surge controller 40 transmits a new position to the anti-surge valve 30, and when an actual modification of the flow through the compressor 10 occurs. This time delay is usually designated as a dead time of an anti-surge response. The dead time may be due to a non-linearity of the anti-surge valve's actuator, and delays along fluid transport pipes of the anti-surge loop 20. The dead time effects include a reduction of a stability margin, and poor dynamic performances in order to preserve stability of the system (e.g., a low gain setting).
A schematic diagram of a conventional anti-surge controller used in the conventional system 1 is illustrated in FIG. 2. The conventional anti-surge controller 50 interacts with a process 60. In FIG. 2, the process 60 stands for a system including a compressor (e.g., 10 in FIG. 1) and an anti-surge loop (e.g., 20 in FIG. 1) with an anti-surge valve (e.g., 30 in FIG. 1). The anti-surge controller 50 receives information about the operation of the system (collectively designated as field measures) from the process 60.
A computing block 70 of the anti-surge controller 50 calculates a value of an anti-surge parameter using the field measures received from the process 60. The value of the anti-surge parameter is proportional to a value of the total flow through the compressor, which is a sum of an input flow and the recycled flow of fluid. For example, the anti-surge parameter may be proportional to hs×Psd/Ps where hs is a differential pressure through a flow element located close to the suction of the compressor, Psd is a design value of a suction pressure and Ps is an actual value of the suction pressure.
An add/subtract block 80 compares the calculated value of the anti-surge parameter with a margin, which is a value of the anti-surge parameter considered safe for the operation of the system. A proportional plus integral (PT) controller 90 determines and outputs the new position to the anti-surge valve. Prior to the PI controller 90 a dead-band error filtering block 85 filters the signal input to the PI controller 90 in order to avoid signal noise impacting the new position towards the anti-surge valve. After the PI controller 90 outputs the new position to the anti-surge valve, a rate limiter 95 may adjust the new position to ensure that the position does not vary at a rate larger than an operational safe value.
Shortly after the position has been changed, the field measures do not reflect the change due to the dead time. Therefore, the conventional anti-surge controller may over-correct or under-correct the position sent to the anti-surge valve. No prevention or correction for dead time is provided in the conventional controller. FIG. 3 is a graph of the anti-surge parameter 110 and the position 120 versus time, illustrating the oscillations due to over-correcting of under-correcting the position of the valve.
The presence of the dead time, which can be in the range of 1-10 s, renders the anti-surge loop 20 unstable. In order to avoid this instability, the system 1 may be operated based on an additional margin with respect to the anti-surge line (which may be a line in a graph of a pressure ratio across the compressor versus the flow through the compressor at which a surge phenomenon occurs), but this manner of operation reduces the compressor's operating envelope.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks.